1. Field of Invention
The present invention is related to an improvement of luminous efficiency of a gallium nitride (GaN) based light-emitting diode (LED). in particular, the present invention is related to a GaN LED with a metal micro-structure as a light extraction layer and a manufacturing method for the same.
2. Related Art
Semiconductor light-emitting diodes (LEDs) have been developed for several decades and the luminous efficiency thereof plays a key role in whether LEDs may be further applied in lighting facilities generally used in ordinary living. Therefore, LED research, for the past decades, has been focused on improvement of luminous efficiency. Generally, luminous efficiency varies with the following factors: semiconductor material adopted, device structure devised, transparency of material used, total reflection existed, etc.
Of the semiconductor LEDs, a gallium nitride (GaN) based material may be the most commonly used. To let the GaN-based material irradiate light, a voltage or a current has to be applied to the corresponding LED. To apply a voltage or a current to the LED, a pair of positive and negative electrodes are disposed on the LED structure.
The positive electrode is also called a p-electrode while the negative electrode is also called an n-electrode, since charges provided by the p-electrode first flow into a p-type semiconductor material layer and charges supplied by the n-electrode first flow into an n-type semiconductor material layer. The p-type electrode is where positive charges flow into the LED structure, and holes are carriers for conductivity. On the other hand, the n-type electrode is where negative charges flow into the LED structure, and electrons are carriers for conductivity. Owing to the poorer mobility of the hole carriers as compared to the electron carriers, a current spreading layer is generally disposed under the p-type electrode so that the hole carriers may be uniformly distributed in the p-type semiconductor layer. In this case, electric force lines between the p-type electrode and the n-type electrode may also be uniformly distributed so as to enhance excitation of light in the LED. The afore-mentioned current spreading layer may be any suitable material, and a Ni/Au double layered structure is the most commonly used. Referring to FIG. 1, which illustrates an LED structure 10, the LED structure 10 comprises a substrate 11, a buffer layer 12, an n-type GaN-based layer 13, a semiconductor active layer 14, a p-type GaN-based layer 15, a p-type semiconductor layer 16, a current spreading layer 17 and a p-type electrode 18. The process thereof may be found in ROC patents 558848, 419837, etc. In the figure, the current spreading layer 17 is disposed between the p-type semiconductor layer 16 and the p-electrode 18 and used to distribute uniformly positive charges thereon and then enter into the p-type semiconductor layer 16.
However, a serious total reflection issue is closely related to the current spreading layer since the current spreading layer has flat surface and thus reflects light back to the LED structure and has a poor light extraction. Some technologies for roughening the current spreading layer are provided. For example, roughening structures are disposed where the emitted light is output so that most emission angles of the emitted lights are smaller than a critical angle, which is defined by Snell's Law. These roughening structures are generally shaped as hemispheres or truncated pyramids. However, these roughening shapes are hard to form and may be expensive.
Other roughening technologies are available. For example, an etch process is applied onto the upper flat surface of the LED to form small, roughened facets on the flat surface so that most emission angles of emitted light may output without reflection to the LED structure. Such a roughening method comprises a process of randomly etching a surface. For example, particles are deposited on the surface and then used as masks in the random etching. However, there are at least two major disadvantages. First of all, some small islands may exist in the p-type electrode. Since the lower parts of the island structures do not contact the p-type electrode contact, no light will be emitted by these portions and the total light output is reduced. Second, since the upper surface of the LED structure is very close to the light-generating area below, the light generating area may be very likely broken.
In view of the disadvantages of the prior GaN-based LED, there is a need to provide a GaN-based LED with high light extraction efficiency.